Processing of refined chemical pulp into viscose by adding cation active sulphonium compounds



Patented Oct. 7, 1947 PROCESSING OF REFINE!) CHEMICAL PULP INTO. VISCOSE BY ADDING CATION AC- TIVE SULPHONIUM COMPOUNDS 4 Paul Henry Schlosser and Kenneth Russell Gray,

Shelton, Wash., assignors to Rayonier Incorporated, Shelton, Wash., a corporation of Delaware ' No Drawing. Application July 18, 1943,

Serial No. 495,042

1 This invention relates to 'the production of cellulosic products, and has for its general oblest the provision of certain improvements in The invention also provides, as a new article of manufacture, a chemically prepared wood pulp product having incorporated therein a cationactive sulphonium' compound.

Chemically prepared wood pulps are exten sively used in the industrial arts for the production of such cellulosic products as rayon and other synthetic fibers, nitrocellulose, cellulose acetate, cellulose ethers, Cellophane and similar cellulosic films or sheets, etc.- Wood pulp is commonly prepared and marketed in sheet form, and comminution or shredding is customarily one of the initial steps in its subsequent processing. When 16 Claims. (Cl. 260-217) wood pulp is used in bulk form, either wet or dry,

shredding or comminution is frequently a step involved in the subsequent processing. Since such subsequent processing usually involves the conversion of the cellulose of the wood pulp to some cellulose derivative and the solution of the derivative in a suitable solvent, the purpose of shredding is to break down the sheet into a flufiy mass or crumb in which the individual fibers will be suitably exposed to the action of the derivative-forming reagent or reagents. Thus, for example, in the production of viscose, the wood pulp sheet is customarily steeped in a caustic soda solution to form alkali cellulose, and the alkali cellulose, after pressing and while still in sheet form, is shredded or comminuted to properly prepare the cellulose fibers for the xanthating reaction with carbon bisulfide. The resulting sodium cellulose xanthate is dissolved in dilute caustic soda to form the solution commonly known as viscose, which is opacified if desired, filtered, and spun into filaments.

Normal dissolving wood pulps in present use consist mainly of cellulose, but contain appreciable amounts of non-celluloslc impurities, such as hemi-celluloses, fats, resins, waxes, etc. One of the main objects in the manufacture of a;'

highly refined dissolving pulp is to remove as much as possible of the non-cellulosic impurities,

so that a whiter, purer pulp results, which is capable, in the manufacture of rayon, of producing a higher grade yarn.

We find. however, that not all of the noncellulosic impurities which can be removed are undesirable, and in fact, certain of such impurities, normally present in small amounts, are highly. beneficial in aiding the processing of the pulp into viscose, especiallyas regards the step of shredding the alkali cellulose. The beneficial impurities which aid the shredding operation are probably of the nature of fiber lubricants which permit the steeped and pressed pulp to be thoroughly shredded into ,a fiufly condition more readily and without mechanical damage to the alkali cellulose fibers, which would cause them to react incompletely with carbon bisulfide. In a pulp which has not been highly refined, most of these beneficial impurities constitute a portion of the materials removable by organic solvents, as ior example, ether, benzene, alcohol, etc. These beneficial impurities, often loosely termed resins, are mainly of the nature of waxes, fats and resins, the latter often beingpresent in a relatively small amount.

In theory, the problem of making a good pulp could be solved by removing all the undesirable impurities while retaining those impurities which the fibers or otherwlsei In practice, such a cleaniacilitate the shredding operation by lubricating cut separation is difllcult to accomplish directly. We have discovered that better results are obtainable by removing most or all of the impurities, including those which serve beneficially as lubricants to the alkali cellulose fibers during shredding, and then adding to the pulp or to the alkali cellulose prior to the completion of shredding a sufllcient amount of a material of a class entirely diilerent from the natural impurities originally present, and which greatly aids the steps of shredding and filtering.

. White, highly purified or refined wood pulps are very advantageous for the production of'high grade raypn yarns of superior strength and color, and f r this reason, are highly esteemed by the trade. Such highly/refined pulps are in general characterized b having an ether extract of about 0.15% or less where such values refer to the amountot natural ether-extractable material leitin the pulp after the purification processes. While our invention is particularly applicable to such highly refined pulps containing not more than about 0.15% of ether-extractable material, and increasingly so as the ether extract approaches zero, it may be applied with certain advantages to the processing of normaldissolving pulps containing substantially more than 0.15% of ether-extractable materialyalthough" such pulps do not generally. yield the highest grade yarnsand their processing into shredded alkali cellulose is not accompanied by any-particulardifiiculty.

The surface-active portion of the beneficialimpurities, naturally present as such in the.

refined pulps or formed during the processing,

class known are predominately of the general I as anion-active materials. a g 7 We have discovered that a class of cationactive sulphonium compounds give important results in the processing of chemical pulp. In

general, the compounds which we have discovered for use in our invention are much more eilective than the beneficial portion of the impurities naturally present. Accordingly, the cation-active compounds can be used in very minute quantities. go This is doubly advantageous, because such additions of materialsare inexpensive and also because the very pure pulp'treated with the compounds is substantially free. from non-cellulosic materialsdue to the minuteness of the quantity 5 of thecompounds required. Based on these dis- 7 coveries, our invention involves improving one or more of the processing steps in the production of cellulosic products from-chemically prepared wood. pulp by carrying out one or more of the 3b processing steps in the presence of a cationactive sulphonium compound. When applied to the preparation and processing of viscose derived from chemically prepared wood pulp, the invention particularly involves carrying out the shredding of the alkali cellulose, or at least the final stage of shredding, and the filtration of the viscose in the presence of the cation-active sulphonium compound.

The cation-active sulphonium compounds used 40 incur invention are of the cation-active class, which means that the group'or groups responsible for surface activity are in the positive instead of the negative ion of'the molecule. As

a class, cation-activesulphonium compounds are superior in a number of ways to the anion-active compounds for use in the viscose process prior to the completion of shredding, especially as regards treatment of the original pulp. A great many cation-active sulphonium compounds are o soluble in all concentrations of caustic soda that wouldbe encountered in any stage ofthe viscose process; This caustic soda solubility has certain V advantages as regards the final viscose solution 7 but would seem to preclude or limit their use to steps subsequent to steeping or to the use in the steeping caustic which would, however, be more expensive. We have made the important discovery that cation-active sulphonium compounds possess the remarkable and important '30 property of being adsorbed and held very firmly by-cellulosic materials in the presence of steeping caustic soda solutions.

The closeness of the association and the firmness. with which the sulphonium compound is 5 held to the cellulose even in strong caustic soda solutions gives'important. results. Thus, even though a; cation-active sulphonium compound may be completely soluble in 18% caustic soda in the absence of'cellulose, on the other hand when pulp is impregnated with the compound and then steeped in 18% caustic soda, as in the first step of the viscose process, the compound is substantially retained by thepulp' due to the 7,

strong. attraction for the cellulose. a

Anothergreat practical advantage in, using cationeactive sulphonium compounds for treating the pulp itself results from the substantivity of these materials toward cellulose. Thus, the

pulp may be treated with these compounds prior to sheet formation as,rfor example, in the form of adilute suspensionof pulp inwater in a stock 3 chest without the necessity of very completely recirculating the white water. This is in con-1 trast to'the practical limitations under which anion-active compounds would usually have to be used when being applied to the pulp. With anion-active compounds it is generally necessary to' apply these after the sheet has been formed and the greater part of the water has been rers i g the mechanical dehydration pursuant to sheet formation.

sulphonium compounds are organic sulfur compounds wherein the sulfur atomhas three covalent bonds and one electrovalent'bond and wherein the three covalent bonds each link the sulfur to a carbon atom forming amonovalent cation which in turn is joined by the electrovalent bond to an anion. If such compounds contain at least one lipophylic group of sufiicient magnitude attached to the sulfur by one or more of the three covalent linkages. they will be cation active. .By

a lipophylic group is meant a group having 7 substantially a hydrocarbon character and having a definite afiinity for fats and oils. Among the groups having the-strongest lipophylic character,

are aliphatic hydrocarbon radicals such as are derived from fats, waxes, mineral oils, etc. The aliphatic hydrocarbon radicals which may beiused 'to-impart surface activity include straight chain, branched chain, saturated and unsaturated radi Other groups such as cyclic hydrocarbon groups or groups containing a small proportion of g other elements, than carbon and hydrogen are,

cals.

however, not excluded.

It will be understoodthat mama. is not of particular consequence since it doesnot impart any appreciable degree of surface activity to the molecule. anion such as iodide, bromide, chloride, methyl sulfate, bisulfate, sulfate, toluene sulphonate, hy-' droxide, etc. i

In regard to the use of aliphatic hydrocarbon radicals to inducesurface activity. suitable 1ipophylic groups will includethose aliphatic hy- Y drocarbon radicals havingv at least- '7 carbon atoms. There is a, practical upper limit for these or for any other type of lipophylic, surface-ac tive inducing radical in that the compounds must be at least dispersible (and preferably soluble) in water or solutions of alkali metal hydroxide.

' When the lipophylic group attached to the sulfur is not a hydrocarbon group. there is also the provision that the cation must be substantially stable in solutions of alkali metal hydroxide at the temperatures normally encountered in the viscose process up to and including the step'of. shredding.

v The most preferred class of cation-active sulphonium compounds are those which have" attached'to the sulfur a normal primary aliphatic. hydrocarbon radical. either saturated or unsat- Such anion could be any innocuous urated, with from 7-20 carbon atoms and two hydrocarbon groups with from 1-6 carbon atoms.

Among the sulphonium compounds which we have used successfully in practicing our invention arethe following: Lauryl methyl ethyl sulphonium iodide, lauryl methyl ethyl sulphonium methyl sulfate, lauryl diethyl sulphonium ethyl sulfate, cetyi dimethyl sulphonium methyl sulfate, cetyl methyl ethyl sulphonium methyl sulfate, decyl ethyl methyl sulphonium methyl sulfate, lauryl hexyl methyl sulphonium methyl sulfate, dilauryl methyl sulphonium methyl sulfate, oleyl methyl ethyl sulphonium methyl sulphate, octadecyl methyl ethyl sulphonium methyl sulfate, (2-ethyl hexyl) methyl ethyl sulphonium methyl sulfate. An example of a compound which we have found to be particularly effective and practical in improving the steps of shredding and filtering is lauryl methyl ethyl sulphonium methyl sulfate.

We have also used successfully in our invention the sulphonium compound which readily forms on heating equimolar quantities of methyl ethyl sultide, lauryl bromide, and stannic chloride.

The compounds of the invention may be prepared by any of the usual methods for preparing sulphonium compounds. The most common method is to react a thlo ether with an estermost commonly an alkyl ester of a strong acid such as hydriodic, hydrobromic, hydrochloric or sulfuric acid. Other methods of preparation are of course possible, as, for example, formation of the reacting ester in situ as given in British Patcut 509,871. It will be noted that in our preferred group of compounds the lipophylic surface-active inducing radical is a normal primary aliphatic hydrocarbon radical, either saturated or unsaturated, within the range from 7 to carbon atoms. Commercially, the main source of such long chain normal primary aliphatic hydrocarbon radicals is the vegetable and animal fats and oils. Such fats or oils may be utilized in preparing the compounds of our invention in various ways, forexample, by catalytic reduction by hydrogen to the fatty alcohols. The fatty alcohols may then be utilized by converting them according to well-known practices,-via such compounds as halides or mercaptans, to thio ethers which may then be converted to sulphonium compounds. The long chain fatty alcohols may, on the other hand, be converted to alkyl halides or other alkyl esters which may then be reacted with low or high molecular weight thio ethers to give sulphonium compounds. In preparing the compounds used in the invention, it is not necessary to use pure long chain alkyl compounds. Indeed, higher molecular weight compounds prepared as indicated from fats and oils are generally, if not always, offered in commercial quantitles in the form of mixtures of different hydrocarbon chain lengths. That is, for example, in place of using pure lauryl bromide in the preparation of a lauryl methyl ethyl sulphonium compound, technical lauryl bromide such as may be prepared from technical lauryl alcohol may be used. Such technical lauryl bromide will be essentially a mixture of Ca, C10, C12, C14 and C10 carbon length chains with the C12 (that is, the lauryl) chain predominating. Sulphonium compounds prepared from such a technical mixture of albl bromides function in our invention, we find, in a substantially identical manner as sulphonium compounds prepared from the individual pure bromides. Actually, however, when using such a mixture in which lauryl bromide predomimites and containing both lower and higher chain lengths, the result is almost the same as if pure lauryl bromide were used. Again it is obvious that, if the source of the lipophylic surface-active inducing hydrocarbon radicals is petroleum products, it will also usually be commercially most practical to use mixtures of hydrocarbon radicals representing a certain spread of chain lengths (preferabl narrow) rather than pure products. Also in place of using sulphonium compounds prepared from open chain thlo ethers as in the specific examples given above, sulphonium compounds prepared from cyclic thlo ethers such as tetramethylene sulfide may be used.

While highly refined wood pulps are advantageous for the production of high grade rayon,

yarns of superior strength and color, the reaction of the shredded alkali cellulose from such pulps with carbon bisulfide is frequently incomplete. This impairment in the xanthating activity of the shredded alkali cellulose is due to some damage to the fibers during the shredding operation or to incomplete comminution or to formation of compressed fiber bundles. In the case of the conventional shredder having revolving blades coacting with a stationary saddle bar,-the tendency for such damage is especially great if the clearance between the revolving blades and the saddle bar is a little less than the correctvalue. In an extreme case, with a very highly refined pulp, shredding, even under optimum conditions, may produce a shredded pulp which xanthates less completely than if the alkali cellulose were not shredded at all. In other cases, it is possible to obtain reasonably satisfactory shredding of the alkali celluulose from highly refined pulps by adjusting the shredder clearance and by experimenting to find the optimum shredding time for the particular pulp and particular shredder. In this manner it is'possib-le in some cases to obtain almost as complete a degree of xanthation of the shredded alkali cellulose from a highly tions makes the shredding of highly refined pulps too sensitive for satisfactory commercial practice. -This sensitivity to damage during shredding of the alkali cellulose from highly refined pulps isovercome, in our invention, by carrying out the shredding operation in the presence of a cation-active sulphonium compound which may be added to the pulp prior to shredding or to the alkali cellulose prior to the completion of shredding. The improvement in shredding produces a shredded alkali cellulose which reacts substantially completely with carbon bisulflde, and the resulting viscose is comparatively free of unreacted fibers and filtration proceeds more rapidly and economically.

In the usual viscose process the sheets are first subjected to a steeping step to convert the cellulose to alkali cellulose, and the pressed sheets of alkali cellulose are then shredded to form a fiuffy mass of fibers. The fiufly mass is xanthated, dissolved in dilute caustic soda and the solution commonly known as viscose filtered to remove undissolved fibers and gel-like materials, and ripened to impart the desired properties for satlsfiltration of the viscose is to incorporate the sulphonium compound in the wood pulp. This.

the latter case, the white water need not be recirculated in order to prevent considerable loss of the agent when eliminating water in sheet formation in view of the substantivity of the products toward cellulose. In any case, there is produced a chemically prepared wood pulp product having a cation-active sulphonium compound incorporated therein. When the cation-active sulphonium compound is so incorporated in the wood pulp, by the manufacturer thereof, the pulp comes to the rayon manufacturer in a form calculated to secure the full advantages of the invention in the preparation and processing of the viscose into high grade rayon yarns.

The amount of the cation-active sulphonium compounds used in the practice of the invention is relatively small, ranging from 0.01 to 0.20%, and preferably from 0.02 to 0.10%, by weight on the bone dry weight of the wood pulp used, when the agent is incorporated in or added to the pulp or to the alkali cellulose, So far as the objectives of the invention are concerned, there is little if anyimprovement by increasing the amount of sulphonium compound above 0.20% and such higher amounts frequently give rise to certain disadvantages. These disadvantages include the causing of excessive softness in the sheet, resulting in mechanical difficulties in steeping, excessive ball formation in xanthation, diiliculties in the dissolving operation due both to the excessive ball formation in the xanthating step and due to excessive foaming in the viscose solution. Also there may be considerable difficulty in obtaining a completely de-aerated viscose which is absolutely necessary for satisfactory spinning.

Higher concentrations of the sulphonium com-' pounds also unduly lower the surface tension of the viscose, thus changing the coagulating conditions so that the viscose cannot be satisfactorily spun by standard methods, causing the filaments to break and the thread to stick to the godet wheels or thread guides.

While it is our preferred practice to incorporate the cation-active sulphonium compound in the wood pulp, preferably a highly refined pulp containing not more than about 0.15% of ether extractable material, the presence of the compound during the processing steps of shredding may be secured in any other appropriate manner. Moreover, when the pulp is in sheet form, the compound need not be incorporated in every sheet, but may be incorporated in only alternate sheets. It may be added to only a portion of the pulp in whatever form it is marketed. Alternatively, the compound may be sprayed upon or otherwise suitably added to all or a portion of the alkali cellulose prior to shredding or prior to the completion of shredding. However, we believe it will generally be found more advantageous to incorporate the cation-active sulphonium compound in the initial wood pulp product, both as a matter of convenience and economy in preparthe viscose industry,

8 ing and processing the viscose, and because a very uniform distribution of the compound throughout the viscose is easily attained.

When the invention is practiced for effecting the hereinbefore-mentioned improvements in shredding, certain further economies are effected in the subsequent steps of xanthating, dissolving and filtering. In viscose solutions there is usually a certain amount of undissolved fibers and gellike material due to the incomplete reaction of the cellulose with the carbon bisulfide during xanthation. Prior to spinning, the viscose solutions are filtered several times to remove these gels and undissolved fibers. 'In the event thatthe viscose solutions contain excessive amounts of undissolved and partially dissolved fibers, filtration is an expensive operation. In such cases the filters become rapidly clogged and the filter media must be changed frequently in order that the viscose will pass through in a reasonable time. Frequent changing of the filter media is expensive, not only as regards consumption of filter cloth but also in view of the very considerable amount of labor involved and also since a certain amount of viscose is lost every time the filter is opened up. Furthermore, when the viscose solutions contain very large proportions of gel-like material, filtration is usually not altogether satisfactory in that some of the smaller gel-like particles tend to pass through the pores of the cloth with adverse effect upon the spinning operation. It has heretofore been the practice in when processing pulps which tend to yield viscose solutions high in undissolved material and having poor filtration properties, to minimize such difiiculties by carrying out the xanthation with amounts of carbon bisulfide considerably in excess of that normally required. Use of excess carbon bisulfide is expensive and in addition is technically undesirable in that it may adversely affect certain properties such as the ripening of the viscose and yarn characteristics. We have found that when processing highly purified pulps which would normally tend to give poorly filtering viscose solutions, the addition of minute amounts of the sulphonium compounds of the invention so im.

proves the shredding operation that the alkali cellulose subsequently 'reacts much more completely with carbon bisulfide and yields a viscose solution very free from undissolved and partially dissolved cellulose particles and having good filtration properties. This result can be accomplished not only without the use of excess carbon bisulfide, but in many cases satisfactory viscose solutions can be obtained using amounts of carbon bisulfide very substantially less than the amounts normally required.

We claim:

1. The method of improving the manufacture of shredded alkali cellulose from refined chemical sulphonium compound at least dispersible in, and whose cation is substantially stable in, solutions of alkali metal hydroxide, said compound being added in a range f rom 0.01% to 0.20% by weight, such percentages being based on the bone,

dry weight of the pulp.

3. As a new article of manufacture, a refined chemical pulp product containing not more than 0.15% of natural ether extractable matter and having incorporated therein a cation-active sulphonium compound at least dispersible in, and whose cation is substantially stable in, solutions of alkali metal hydroxide, the amount of the incorporated sulphonium compound being from 0.01% to 0.20% by weight, such percentages being based on the bone dry weight of the pulp.

4. The method of improving the processing of refined chemical pulp containing not more than 0.15% ether extractable matter into viscose comprising adding to such refined chemical pulp prior to use in the viscose process a cation active sulphonium compound at least dispersible in solutions of alkali metal hydroxide and wherein one of the groups attached to the sulfur by a covalent bond is an aliphatic hydrocarbon chain of 7-20 carbon atoms, said compound being added in a range from 0.01% to 0.20% by weight, such percentages being'based on the bone dry weight of the pulp.

5. The method of improving the manufacture of shredded alkali cellulose from refined chemical pump containing not more than 0.15% ether extractable matter comprising incorporating in the cellulosic material a cation active sulphonium compound wherein one of the groups attached to the sulfur by a covalent bond is a normal primary aliphatic hydrocarbon chain with from 7-20 carbon atoms and two other groups attached to said sulfur by the remaining two covalent bonds are hydrocarbon groups each containing from 1-6 carbon atoms, said compound being added in a range from 0.01% to 0.20%, such percentages being based on the weight of the bone dry pulp.

6. The method of improving the processing of refined chemical pulp containing not more than 0.15% ether extractable matter into viscose comprising adding to such refined chemical pulp prior to use in the viscoseprocess a cation-active sulphonium compound wherein one of the groups attached to the sulfur by a covalent bond is a normal primary aliphatic hydrocarbon chain with from '7-20 carbon atoms and the other groups attached to said sulfur by the remaining two covalent bonds are hydrocarbon groups each containing from 16 carbon atoms, said compound being added in a range from 0.01% to 0.20% by weight, such percentages being based on the bone dry Weight of the pulp.

7. As a new article of manufacture, a refined chemical pulp product containing not more than 0.15% of natural ether extractable matter and having incorporated therein a cation-active sulphonium compound wherein one of the groups attached to the sulfur by a covalent bond is a normal primary aliphatic hydrocarbon chain with from I 20 carbon atoms and the other groups attached to said sulfur by the remaining two covalent bonds are hydrocarbon groups each containing from 1-6 carbon atoms, the amount of the incorpo ated sulphonium compound being from 0.01% to 0.20% by weight on the bone dry pulp.

8. The method of improving the processing of refined chemical pulp containing not more than J10 0.15% ether extractable matter into viscose comprising adding to such refined chemical pulp prior to use in the viscose process lauryi methyl ethyl sulphonium methyl sulfate in a range from 0.01 to 0.20% by weight, such percentages being based on the bone dry weight of the pulp.

9. The method of improving the processing of refined chemical pulp containing not more than 0.15% ether extractable matter into viscose comprising adding to such refined chemical pulp prior to use in the viscose process cetyi dimethyl sulfonium methyl sulfate in a range from 0.01% to 0.20% by weight, such percentages being based on the bone dry weight of the pulp.

10. The method of improving the processing 0! refined chemical pulp containing not more than 0.15% ether extractable matter into viscose comprising adding to such refined chemical pulp prior to use in the viscose process lauryl diethyl sulfonium ethyl sulfate in a range from 0.01% to 0.20% by weight, such percentages being based on the bone dry weight of the pulp.

11. In the processing of refined chemical pulp containing not more than 0.15% ether extractable matter to form viscose, the step which comprises shredding alkali cellulose containing a cationactive sulphonium compound at least dlspersible in, and whose cation is substantially stable in, solutions of alkali metal hydroxide, said compound being present in the alkali cellulose in a range from 0.01% to 0.20% by weight based on the weight of the bone dry pulp.

12. In the processing of refined chemical pulp containing not more than 0.15% ether extractable matter to form viscose, the step which comprises shredding alkali cellulose containing a cationactive sulphonium compound at least dispersible in solutions of alkali metal hydroxide and wherein one of the groups attached to the sulfur by a covalent bond is an aliphatic hydrocarbon chain of [-20 carbon atoms, said compound being added in a range from 0.01% to 0.20% by weight based on the weight of the bone dry pulp.

13. In the processing of refined chemical pulp containing not more than 0.15% ether extractable matter to form viscose, the step which comprises shredding alkali cellulose containing a cation active sulphonium compound wherein one of the groups attached to the sulfur by a covalent bond is a normal primary aliphatic hydrocarbon chain with from 7-20 carbon atoms and the other groups attached to said sulfur by the remaining two covalent bonds are hydrocarbon groups each containing from 1-6 carbon atoms, said compound being added in a range from 0.01% to 0.20% by weight based on the weight of the bone dry pulp.

14. In the processing of refined chemical pul containing not more than 0.15% ether extractable matter to form viscose, the step which comprises shredding alkali cellulose containing lauryl methyl ethyl sulphonium methyl sulfate in a range from 0.01% to 0.20% by weight based on the weight of the bone dry pulp.

15. In the processing of refined chemical pulp,

ethyl sulfonium ethyl sulfate in a ran e from V 0.01% to 70.20% by weight based on the weight v UNITED STATES PATENTS W of the bone dry pulp' Number Name Date 2,331,936 Schlosser et a1 Oct.19, 1943 5 17 ,196 P012114 1 Nov. *1, 193,9 2,121,823 Pig'gott June 28, 1938 REFERENCES CITED FOREIGN PATENTS The fol lowing references are of record in the m Number 9 Country r Dete file of this patent: 521,727 Great Britain May 29, 1940 V 

